Not applicable.
The invention relates in general to data signal processing and, in particular, to a hybrid circuit within a modem for coupling to a telephone line to improve received signal discrimination.
Most users of data communications services access data communications networks (e.g. the Internet) using dial-up connections established through the Public Switched Telephone Network (PSTN). The PSTN is still substantially an analog communications network designed, before the advent of digital communications, to transmit sounds in the audible range of the human voice.
Digital data is transported across the PSTN by converting the data into an analog signal that is transmitted by varying, or modulating, the frequency, phase, amplitude or other characteristic of a carrier signal. The modulation is performed by a modem attached to a standard telephone line referred to as a xe2x80x9clocal loopxe2x80x9d. When analog signals are received from other modems in the PSTN, the receiving modem performs an opposite function by demodulating the received analog signal to convert it back into digital data.
Analog signals are simultaneously transmitted and received by the modem through the local loop. Thus the local loop carries a mixed signal that includes a combination of both transmit (Tx) signals being sent by the modem, and receive (Rx) signals being received by the modem. The modem must therefore separate the Tx and Rx signals so that a substantially uncorrupted Rx signal can be supplied to the demodulation portion of the modem. The separation of the Tx and Rx signals is commonly performed using a circuit known as a hybrid circuit, which is located between the modem""s modulation/demodulation circuits and the local loop.
As is well known in the art, a classical hybrid circuit includes a line driver that energizes a line transformer through an impedance (typically a resistor) chosen to provide an appropriate termination impedance for the local loop (which is usually a complex value). In order to separate the Rx signals from the mixed signal on the local loop, the signal line is tapped at a first tap point (proximal the line transformer) and supplied to a summing circuit (typically a differential amplifier). The Tx signal is also supplied to the summing circuit by tapping at a second tap point in a compensation network. The purpose of the compensation network is to provide a branch line from the line driver (source of Tx signals) in which the strength of the Tx signal is directly proportional (preferably equal) to the Tx signal strength at the first tap point, while the Rx signal strength is strongly attenuated. The summing circuit can then isolate the Rx signal from the mixed signal by finding a difference between the signals obtained at the first and second tap points.
In practice, the effectiveness of this classical hybrid circuit depends on the match between the impedance of the line (ZLINE)and that of the compensation network (ZBAL). In particular, if ZBAL=ZLINE (where k is a constant) at all frequencies, then the summing circuit will completely remove the Tx signal from the mixed signal, and none of the Tx signal will xe2x80x9cleakxe2x80x9d past the summing circuit.
However, the impedance of the local loop cannot be matched by a generic compensation network, because it depends on several factors (e.g. loop length, physical properties, loop topology, and the presence of bridged taps, etc.) that are termination-specific. Consequently, in practice, ZBAL less than  greater than k*ZLINE and at least some of the Tx signal will leak past the summing circuit. This Tx signal leakage appears as noise in the Rx signal supplied to the demodulator of the modem. This noise (Tx leakage) may include distortion components generated by the line driver, and may also cause distortions in the demodulator that fall in the Rx signal frequency band and cannot be separated from the useful Rx signal.
Additionally, at a minimum usable Rx signal strength, common-mode noise originating from multiple sources (e.g., Vcc, ground, capacitive coupling, etc.) may rival the Rx signal strength. If this occurs, the Rx signal integrity will be severely compromised, resulting in an unacceptably low signal-to-noise ratio.
It is known that Tx signal leakage can be reduced by inserting filters between the tap points and the summing circuit. The filters can be tuned to attenuate signals in the Tx frequency band, without attenuating the Rx signals. While such filters reduce Tx signal leakage, the filter circuits may induce the same distortion problems discussed above, resulting in distortion components (within the Rx frequency band) that are applied to the inputs of the summing circuit. As is well known, those distortion components cannot be separated from the useful Rx signal. In addition, the filter circuits do not address problems associated with common-mode noise.
As taught in U.S. Pat. No. 5,822,426 (Rasmus et al.), which issued Oct. 13, 1998, the problem of common-mode noise can be addressed by using a balanced pair of line drivers, respectively generating complementary Tx+ and Txxe2x88x92 signals (or by inverting the mixed signal derived at the second (Tx) signal tap). This effectively eliminates common-mode noise supplied to the input of the summing amplifier that extracts the Rx signal. However, Rasmus et al. do not address the problems of Tx signal leakage or distortion.
Accordingly, there remains a need for an improved circuit for coupling a modem receiver (demodulator) to a signal line that carries transmit and receive signals simultaneously.
It is therefore an object of the present invention is to provide a directional coupler having high common-mode noise rejection and low transmit signal leakage, while substantially avoiding distortion of components mixed with a received signal.
Accordingly, an aspect of the present invention provides a directional coupler for coupling a signal receiver to a communications circuit having a signal line adapted to simultaneously carry a transmit (Tx) signal sourced from a line driver circuit and a receive (Rx) signal and a compensation network connected to an output of the line driver for balancing an impedance of the signal line. The directional coupler circuit comprises a primary network connected to the communications circuit and having a respective primary network output; an amplifier having a respective amplifier input connected to the primary network output, and a respective amplifier output connected to the receiver; and a feedback network connected between the amplifier input and the amplifier output. The primary network, the amplifier, and feedback network are arranged to provide a virtual ground at the amplifier input.
A further aspect of the present invention provides a modem comprising: a communications circuit for coupling a signal source of the modem to a telecommunications network, the communications circuit comprising a signal line adapted to simultaneously carry a transmit (Tx) signal sourced from a line driver circuit and a receive (Rx) signal having a frequency differing from that of the Tx signal and a compensation network connected to an output of the line driver for balancing an impedance of the signal line; and a directional coupler for coupling a signal receiver of the modem to the communications circuit. The directional coupler circuit comprises: a primary network connected to the communications circuit and having a respective primary network output; an amplifier having a respective amplifier input connected to the primary network output, and a respective amplifier output connected to the receiver; and a feedback network connected between the amplifier input and the amplifier output. The primary network, the amplifier, and feedback network are arranged to provide a virtual ground at the amplifier input.
Preferably, the primary network and feedback network are adapted to cooperate to provide a notch filter characteristic of the directional coupler circuit, the notch filter characteristic having a center frequency substantially corresponding to a frequency of the Tx signal. Alternatively, the primary network and feedback network can be adapted to cooperate to provide a 2nd order chebychev filter characteristic.
In an embodiment of the invention, the primary network comprises: a first input connected to the signal line for receiving a mixed signal; a second input operatively connected to the compensation network for receiving a compensation signal; and a filter network connected between the first and second filter inputs and the primary network output.
In an embodiment of the invention, the filter network comprises a partially-split 4-port RC network connected between the first and second inputs and the primary network output. Preferably, the partially-split 4-port RC network comprises: an output portion comprising a resistor and a capacitor connected in parallel to the primary network output; and a pair of input portions connected between respective ones of the first and second inputs and the output portion, each input portion comprising a resistor connected in series with the resistor of the output portion, and a capacitor connected in series with the capacitor of the output portion. Still more preferably, the partially-split 4-port RC network further comprises: a resistor connected between ground and a junction between the capacitors of the input and output portions; and a capacitor connected between ground and a junction between the resistors of the input and output portions.
In another embodiment of the invention, the primary network comprises a 4-port RC network connected to the first and second inputs via respective first and second resistances. Preferably, the 4-port RC network comprises first and second branches connected in parallel, the first branch comprising a pair of series connected resistors and a capacitor connected between ground and a junction between the series connected resistors; and the second branch comprising a pair of series connected capacitors and a resistor connected between ground and a junction between the series connected capacitors.
In a still further embodiment of the invention, the primary network comprises: a first 4-port RC network connected between the first input and the primary network output; and a second 4-port RC network connected between the second input and the primary network output. Preferably, each 4-port RC network comprises first and second branches connected in parallel, the first branch comprising a pair of series connected resistors and a capacitor connected between ground and a junction between the series connected resistors; and the second branch comprising a pair of series connected capacitors and a resistor connected between ground and a junction between the series connected capacitors.
In an embodiment of the invention, the feedback network comprises first and second branches connected in parallel, the first branch comprising a pair of series connected resistors and a capacitor connected between ground and a junction between the series connected resistors; and the second branch comprising a capacitor.
In an embodiment of the invention, the primary network output comprises a controllable gain stage. Preferably, the controllable gain stage comprises an attenuation network connected in parallel with a gain control switch. The attenuation network is preferably adapted to attenuate a signal substantially without altering the filter characteristic of the directional coupler. The attenuation network can comprise a pair of series connected resistors, and a capacitor connected in parallel with one of the pair of series connected resistors. Preferably, the gain control switch is adapted to selectively bypass the attenuation network.